This type of semiconductor pressure sensor is equipped with a monocrystal silicon substrate as a semiconductor substrate whose one face corresponds to the (110)-face, and a pressure detecting diaphragm formed on one face of the monocrystal silicon substrate (for example, see JP-A-2001-356061 (page 3, FIG. 1): Patent Document 1).
Such a semiconductor pressure sensor is manufactured as follows. That is, a monocrystal silicon substrate in which the face direction of one face corresponds to the (110)-face is prepared, an etching mask is disposed at the one face side of the monocrystal silicon substrate, anisotropic etching is performed on the monocrystal silicon substrate from the one face thereof to form a recess portion in the monocrystal silicon substrate, and then a diaphragm for receiving pressure is formed at the bottom surface side of the recess portion in the monocrystal silicon substrate.
Here, strain gage resistors constituting a bridge circuit for outputting a detection signal in connection with strain of the diaphragm are formed in the diaphragm. The strain gage resistors are formed by subjecting doping, diffusion or the like to the monocrystal silicon substrate before the anisotropic etching is carried out.
Here, the strain gage resistors comprise a pair of center gages disposed at the center portion of the diaphragm so as to extend along the <110> crystal axis direction, and a pair of side gages disposed to be nearer to the peripheral portion of the diaphragm than the center gages (see JP-A-11-94666 (page 11, FIG. 15): Patent Document 2).
Here, FIG. 4 is a diagram showing the arrangement of strain gage resistors Rc1, Rc2, Rs1, Rs2 on the diaphragm 30 located on the principal surface of the monocrystal silicon substrate 10 of the semiconductor pressure sensor using the monocrystal silicon substrate described above. Two crystal axes <110> and <100> which are mutually orthogonal to each other structurally exist on the (110)-face corresponding to the principal surface of the monocrystal silicon substrate 10.
Here, sensitivity to stress occurring in the <110> crystal axis direction is much larger in piezo-resistance coefficient than sensitivity to stress occurring in the <100> crystal axis direction, so that not the stress occurring in the <100> crystal axis direction, but the stress occurring in the <110> crystal axis direction is used to detect the stress on the (110)-face.
Only one direction of <110> exists on the (110)-face, and thus when higher output is required to be achieved with respect to a crystal axis having higher sensitivity, the strain gage resistors Rc1, Rc2, Rs1, Rs2 must be necessarily arranged as shown in FIG. 4.
That is, the center gages Rc1, Rc2 disposed to be deviated to the center of the diaphragm 30 along the <110> crystal axis direction, and the side gages Rs1, Rs2 disposed to be nearer to the peripheral portion of the diaphragm 30 than the center gages Rc1, Rc2 are provided, and these four strain gage resistors constitute a bridge circuit to detect the stress occurring in the <110> crystal axis direction.
Specifically, the resistance value of the center gage Rc1 is set to RA, the resistance value of the center gage Rc2 is set to RD, the resistance value of the side gage Rs1 is set to RB and the resistance value of the side gage Rs is set to RA, and these strain gage resistors are connected to one another in series to form a rectangular closed circuit, thereby forming a Wheatstone bridge as shown in FIG. 5.
In the bridge circuit 100 shown in FIG. 5, the strain of the diaphragm 30 occurs as the variation of each of the strain gage resistors RA, RB, RC, RD under the state that a DC constant current I is applied from an input terminal Ia to Ib, and a voltage (detection signal) having the level corresponding to a detected output, that is, midpoint potential Vout is output between an output terminals Pa and Pb.
As disclosed in the Patent Document 1, the semiconductor pressure sensor as described above is, not shown, normally designed so that a glass seat is attached to a monocrystal silicon substrate 10 by anode bonding or the like.
Since the monocrystal silicon substrate 10 and the glass seat are different from each other in thermal expansion coefficient, thermal stress occurs between them when the temperature varies, and this thermal stress is transmitted to the strain gage resistors Rc1, Rc2, Rs1, Rs2 on the diaphragm 30. Here, the thermal stress applied to the center gages Rc1, Rc2 and the thermal stress applied to the side gages Rs1, Rs2 are greatly different from each other because of the positional difference therebetween on the diaphragm 30.
As a result, the difference between the thermal stress applied to the side gages Rs1, Rs2 and the thermal stress applied to the center gages Rc1, Rc2 is output as a noise. The difference in thermal difference is dependent on the temperature, varies non-linearly, so that the temperature characteristic of the offset of the output has a curved line with respect to the temperature.
Accordingly, in the temperature characteristic of the offset of the output, a difference occurs between the slope of the offset with respect to the temperature from the room temperature to a high temperature and the slope of the offset with respect to the temperature from a low temperature to the room temperature. This difference is referred to as TNO (Temperature Nonlinearity Offset). TNO is an important characteristic for determining the precision of the sensor.
Furthermore, when miniaturization of the semiconductor pressure sensor, that is, miniaturization of the monocrystal silicon substrate 10 is a primary goal, it may be considered to reduce the diaphragm 30 occupying a large area. However, in this case, the following problems occur.
FIG. 6 is a perspective view showing the shape of the diaphragm 30 of a semiconductor pressure sensor when it is viewed from the one-face 11 side of the monocrystal silicon substrate 10. FIG. 7A is a plan view showing the diaphragm shown in FIG. 6, and FIG. 7B is a cross-sectional view taken along a line VIIB—VIIB of FIG. 7A.
As shown in FIGS. 6 and 7A-7B, a recess portion 20 having an opening portion formed in an octagonal shape is formed in the one-face 11 of the monocrystal silicon substrate 10, and an octagonal diaphragm 30 is formed on the bottom surface of the recess portion 20.
Here, the octagonal opening portion of the recess portion 20 is inherited from the shape of the opening portion of the etching mask. In the recess portion 20, four slant faces 21, 22, 23, 24 and vertical faces located between the respective neighboring slant faces are formed as side surfaces from the octagonal opening portion, and the bottom surface of the recess portion 20 is constructed as the octagonal diaphragm 30 through these side surfaces.
Here, a pair of slant faces 21, 22 confronted to each other along the <100> crystal axis correspond to the (111)-face, and a pair of slant faces 23, 24 confronted to each other along the <110> crystal axis correspond to the (110)-face.
The recess portion 20 as described above can be formed by forming the etching mask having the opening portion corresponding to the octagonal opening portion of the recess portion 20, of silicon nitride film on the one-face 11 of the monocrystal silicon substrate 10 with the CVD (Chemical Vapor Deposition) method or the like, and then conducting anisotropic etching with alkali etching liquid formed of KOH (potassium hydroxide) or the like.
In this case, the anisotropic etching progresses by utilizing the difference between the etching rate in the depth direction of the recess portion 20 and the etching rate of the slant faces, whereby the octagonal diaphragm 30 as shown in FIGS. 6 and 7A-7B is formed.
When miniaturization of the sensor, that is, miniaturization of the monocrystal silicon substrate 10 is aimed in the semiconductor pressure sensor having the octagonal diaphragm 30 as described above, the area of the diaphragm occupying a large area is reduced. If so, it may be considered to reduce the size of the octagonal opening portion of the etching mask.
According to studies of the inventors, however, it has been found that when the opening portion of the etching mask is merely reduced in size under the condition that the thickness of the monocrystal silicon substrate 10 and the thickness of the diaphragm 30 are constant, the shape of the diaphragm 30 becomes a rectangle.
For example, under the condition that the thickness of the monocrystal silicon substrate 10 is set to 300 μm, the thickness of the diaphragm 30 is set to about 10 to 20 μm and the longitudinal and lateral dimensions L (see FIG. 7A) are reduced to less than 620 μm, the shape of the diaphragm 30 becomes rectangular in plan view as shown in FIG. 8 A when the above etching method is used. Here, FIG. 8A is a plan view and FIG. 8B is a cross-sectional view taken along a line VIIIB—VIIIB of FIG. 8A.
That is, the etching rate in the depth direction and the etching rate of the slant faces 21 to 24 are settled in the etching process of the recess portion 20. Therefore, as the size of the diaphragm 30 is reduced, the four slant faces 21 to 22 are linked to one another as shown in FIGS. 8A-8B, and thus the shape of the diaphragm 30 becomes rectangular.
When the shape of the diaphragm is varied from the octagonal shape to the rectangular shape as described above, the difference in thermal stress between the center gage and the side gage is larger as compared with the case when the diaphragm 30 has an octagonal shape. FIG. 9 is a diagram showing an analysis result of the magnitude of thermal stress applied to the center gages Rc1, Rc2 and the magnitude of thermal stress applied to the side gages Rs1, Rs2 by using a finite element method (FEM) when the diaphragm 30 has an octagonal shape and when the diaphragm 30 has a rectangular shape.
As is apparent from FIG. 9, the difference between the thermal stress σc applied to the center gages Rc1, Rc2 and the thermal stress σc applied to the side gages Rs1, Rs2 is larger in the case of the rectangular diaphragm 300 than in the case of the octagonal diaphragm 30. Therefore, the TNO characteristic is degraded for a rectangular diaphragm.